The expression "two is one, one is none" refers to preparedness and being ready for the worst eventuality to happen. Having one [ANYTHING] means that you are hinging your success on a single item, which may or may not fail at the worst possible moment (Murphy was an optimist). Having a backup is always a great idea.
Fobus' Belt Holster for Glock handguns is a good holster in its own right. It's simple, maintenance-free and inexpensive, yet still works exactly as intended and advertised. It offers Level II retention, with a paddle activated by the index finger along the slide of the holstered pistol. At a weight of 2 ounces, this is a lightweight option that won't drag your belt down, or add a lot of extra weight to your "spare gear" bag.
Operation is straightforward, with passive retention offered by friction and active retention as indicated with the paddle. A natural draw stroke, with the finger along the area of the slide of the holstered pistol, releases the mechanism and frees the pistol for presentation. It takes a modicum of practice, but is quickly learned and reinforced with time and effort.